Fix post-merge failure in 5.1-merge 
- Let QUICK_RANGE_INTERSECT_SELECT not make assumption that HA_EXTRA_KEYREAD 
  scans do not touch parts of table->record[0] that refer to fields that are 
  not covered by the used index.
  This assumption is not true for XtraDB (e.g. grep row/row0sel.c for 
  "init null bytes with default values as they might be").